One of many extensively used non-volatile magnetic storage devices is a magnetic disk drive. The magnetic disk drive includes a rotatable magnetic disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. The suspension arm is supported by an actuator that can swing the suspension arm to place the slider with its air bearing surface (ABS) over the surface of the magnetic disk.
When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the magnetic disk. When the slider rides on the air, the actuator moves the suspension arm to position the assembly of the write and read heads over selected data tracks on the magnetic disk. The write and read heads write and read data on the magnetic disk. Processing circuitry connected to the assembly of the write and read heads then operates according to a computer program to implement writing and reading functions.
The write head includes a magnetic write pole and a magnetic return pole, which are magnetically connected with each other at a region away from the ABS, and are surrounded by an electrically conductive write coil. In a writing process, the electrically conductive write coil induces a magnetic flux in the write and return poles. This results in a magnetic write field that is emitted from the write pole to the magnetic disk in a direction perpendicular to the surface of the magnetic disk. The magnetic write field writes data on the magnetic disk, and then returns to the return pole so that it will not erase previously written data tracks.
The read head includes a read sensor which is electrically connected with ferromagnetic lower and upper shields, but is electrically separated by insulation layers from longitudinal bias layers in two side regions. In a reading process, the read head passes over magnetic transitions of a data track on the magnetic disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of the read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track.
A current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor is typically used in the read head. The CPP TMR read sensor includes a nonmagnetic insulating barrier layer sandwiched between a ferromagnetic reference layer and a ferromagnetic sense layer, and the CPP GMR read sensor includes a nonmagnetic conducting spacer layer sandwiched between the ferromagnetic reference and sense layers. The thickness of the barrier or spacer layer is chosen to be less than the mean free path of conduction electrons passing through the CPP TMR or GMR read sensor. The magnetization of the reference layer is pinned in a direction perpendicular to the ABS, while the magnetization of the sense layer is oriented in a direction parallel to the ABS. When passing the sense current through the CPP TMR or GMR read sensor, the conduction electrons are scattered at lower and upper surfaces of the barrier or spacer layer. When receiving magnetic fields emitting from the magnetic transitions of the data track on the magnetic disk, the magnetization of the reference layer remains pinned while that of the sense layer rotates. Scattering decreases as the magnetization of the sense layer rotates towards that of the reference layer, but increases as the magnetization of the sense layer rotates away from that of the reference layer. These scattering variations lead to changes in the resistance of the CPP TMR or GMR read sensor in proportion to the magnitudes of the magnetic fields and cos θ, where θ is an angle between the magnetizations of the reference and sense layers. The changes in the resistance of the CPP TMR or GMR read sensor are then detected by the sense current and converted into voltage changes that are detected and processed as read signals.
The CPP TMR read sensor has been progressively miniaturized for magnetic recording at higher linear and track densities. Its thickness, which defines a read gap, is reduced by utilizing thinner reference, barrier, sense or other layers, in order to increase linear densities. Its width, which defines a track width, is reduced by patterning with an advanced photolithographic tool, in order to increase track densities. Further decreases in the thickness and width of the CPP TMR read sensor are desired in order to further increase the linear and track densities, respectively.